Human sperm competition: testis size, sperm production and rates of extrapair copulations

Mixed model-based eQTL analysis reveals lncRNAs associated with regulation of genes involved in sex determination and spermatogenesis: The key to understanding human gender imbalance

BACKGROUND

An imbalance in the prenatal sex ratio in humans may be due to several factors affecting sperm physiology, including genetic features. In this study, we conducted a transcriptome-wide analysis of expression quantitative trait loci (eQTLs) to identify target genes associated with previously described QTLs associated with gender imbalance.

METHODS

A mixed model explaining polygenic effects by genomic covariance among individuals was used to identify the eQTLs using gene expression and genotype data from 462 European/African individuals.

RESULTS

Eight eGenes were associated with four QTLs (P < 4.00 × 10^-5), with strong associations found (P < 4.00 × 10^-8) between rs2485007 and eGenes ANKRD26P3 (P = 3.40 × 10^-9) and LINC00421 (P = 1.35 × 10^-9). ANKRD26P3 and LINC00421 are both lncRNAs associated with the control of testis-dominant genes PELP1, TAF15, NANOG, TEX14, TCF3, ZNF433, ZNF555, TEX37, FATE1, TCP11, and CYLC2 and Y-linked genes SRY and ZFY, as well as several genes with roles in spermatogenesis (ODF1, SPATC1, SPATA3, SPATA31E1, SPERT, SPATA16, MOSPD1, SPATA24, and SPO11) and sex determination (SOX family genes).

CONCLUSIONS

The above eGenes contribute directly or indirectly to gene regulation for sex determination and spermatogenesis, thereby serving as important functional clues for gender-biased selection.

Self-Deception about Fecundity in Women : Modeling the Burley Hypothesis

Concealed fecundity and extended female sexual receptivity have evolved in some primates, including humans, conferring advantages both within primarily monogamous relationships (e.g., benefits from paternity assurance) and from extrapair liaisons (e.g., better access to good genes). As humans evolved the intellectual capacity for decision-making, women became capable of altering their own fertility. In some circumstances, they may choose to ameliorate risks and responsibilities associated with pregnancy by reducing sexual motivation near the perceived most fecund time of their menstrual cycle. But three findings-a general inability of women to accurately recognize their own intervals of fecundity, high variability in ovulation timing, and unconscious transmission and reception of cues associated with fecundity-constitute a physiological and behavioral syndrome that can be considered self-deception. In this study, I develop a descriptive model to determine implications of the hypothesis that these features of female and male physiology and behavior have been shaped by natural selection in response to female decision-making. My analysis shows that consensus motivation for coitus between partners influences both the importance of variable ovulation date and the probability of conception, under the influence of self-deception. It also identifies priorities for future empirical work.

Molecular insights into the divergence and diversity of post-testicular maturation strategies

Competition to achieve paternity has coerced the development of a multitude of male reproductive strategies. In one of the most well-studied examples, the spermatozoa of all mammalian species must undergo a series of physiological changes as they transit the male (epididymal maturation) and female (capacitation) reproductive tracts prior to realizing their potential to fertilize an ovum. However, the origin and adaptive advantage afforded by these intricate processes of post-testicular sperm maturation remain to be fully elucidated. Here, we review literature pertaining to the nature and the physiological role of epididymal maturation and subsequent capacitation in comparative vertebrate taxa including representative species from the avian, reptilian, and mammalian lineages. Such insights are discussed in terms of the framework they provide for helping to understand the evolutionary significance of post-testicular sperm maturation.

Of mice and women: advances in mammalian sperm competition with a focus on the female perspective

Although initially lagging behind discoveries being made in other taxa, mammalian sperm competition is now a productive and advancing field of research. Sperm competition in mammals is not merely a 'sprint-race' between the gametes of rival males, but rather a race over hurdles; those hurdles being the anatomical and physiological barriers provided by the female reproductive tract, as well as the egg and its vestments. With this in mind, in this review, I discuss progress in the field while focusing on the female perspective. I highlight ways by which sperm competition can have positive effects on female reproductive success and discuss how competitive outcomes are not only owing to dynamics between the ejaculates of rival males, but also attributable to mechanisms by which female mammals bias paternity toward favourable sires. Drawing on examples across different species-from mice to humans-I provide an overview of the accumulated evidence which firmly establishes that sperm competition is a key selective force in the evolution of male traits and detail how females can respond to increased sperm competitiveness with increased egg resistance to fertilization. I also discuss evidence for facultative responses to the sperm competition environment observed within mammal species. Overall, this review identifies shortcomings in our understanding of the specific mechanisms by which female mammals 'select' sperm. More generally, this review demonstrates how, moving forward, mammals will continue to be effective animal models for studying both evolutionary and facultative responses to sperm competition. This article is part of the theme issue 'Fifty years of sperm competition'.

Population-specific, recent positive directional selection suggests adaptation of human male reproductive genes to different environmental conditions

Background

Recent human transcriptomic analyses revealed a very large number of testis-enriched genes, many of which are involved in spermatogenesis. This comprehensive transcriptomic data lead us to the question whether positive selection was a decisive force influencing the evolution and variability of testis-enriched genes in humans. We used two methodological approaches to detect different levels of positive selection, namely episodic positive diversifying selection (i.e., past selection) in the human lineage within primate phylogeny, potentially driven by sperm competition, and recent positive directional selection in contemporary human populations, which would indicate adaptation to different environments.

Results

In the human lineage (after correction for multiple testing) we found that only the gene TULP2, for which no functional data are yet available, is subject to episodic positive diversifying selection. Using less stringent statistical criteria (uncorrected p-values), also the gene SPATA16, which has a pivotal role in male fertility and for which episodes of adaptive evolution have been suggested, also displays a putative signal of diversifying selection in the human branch. At the same time, we found evidence for recent positive directional selection acting on several human testis-enriched genes (MORC1, SLC9B1, ROPN1L, DMRT1, PLCZ1, RNF17, FAM71D and WBP2NL) that play important roles in human spermatogenesis and fertilization. Most of these genes are population-specifically under positive selection.

Conclusion

Episodic diversifying selection, possibly driven by sperm competition, was not an important force driving the evolution of testis-enriched genes in the human lineage. Population-specific, recent positive directional selection suggests an adaptation of male reproductive genes to different environmental conditions. Positive selection acts on eQTLS and sQTLs, indicating selective effects on important gene regulatory functions. In particular, the transcriptional diversity regulated by sQTLs in testis-enriched genes may be important for spermatocytes to respond to environmental and physiological stress.

The relationship of body condition, superoxide dismutase, and superoxide with sperm performance

Sperm competition theory predicts a negative correlation between somatic investment and traits that aid in pre- and postcopulatory sexual selection. Sperm performance is critical for postcopulatory success but sperm are susceptible to damage by free radicals such as superoxide radicals generated during mitochondrial respiration (mtSOx). Males can ameliorate damage to spermatozoa by investing in the production of antioxidants, like superoxide dismutase (SOD), which may act as a mechanistic link to pre- and postcopulatory trade-offs. Some male Australian, color-polymorphic painted dragon lizards (Ctenophorus pictus) possess a yellow throat patch (bib) that females prefer over nonbibbed males and are also more likely to win male–male contests indicating that males with bibs may be better at monopolizing females. We tested whether the sperm performance in nonbibbed males was superior to that of bibbed males. We show that overall sperm performance was not different between the bib-morphs, however, higher mtSOx levels were negatively correlated with sperm performance in bibbed males, but not of nonbibbed males. Blood cell mtSOx levels are negatively correlated with SOD activity in the plasma in all males early in the breeding season but SOD was lower in bibbed males. Nonbibbed males maintain a positive correlation between body condition and SOD activity over time while bibbed males do not. Together, these data suggest physiological associations between body condition, SOD activity, and sperm performance are linked to the expression of a yellow gular patch, which may be related to intrinsic differences in the metabolism of bibbed versus nonbibbed males.

Rapid evolution of testis size relative to sperm morphology suggests that post-copulatory selection targets sperm number in Anolis lizards

Post-copulatory sexual selection is thought to be responsible for much of the extraordinary diversity in sperm morphology across metazoans. However, the extent to which post-copulatory selection targets sperm morphology versus sperm production is generally unknown. To address this issue, we simultaneously characterized the evolution of sperm morphology (length of the sperm head, midpiece and flagellum) and testis size (a proxy for sperm production) across 26 species of Anolis lizards, a group in which sperm competition is likely. We found that the length of the sperm midpiece has evolved 2-3 times faster than that of the sperm head or flagellum, suggesting that midpiece size may be the most important aspect of sperm morphology with respect to post-copulatory sexual selection. However, testis size has evolved faster than any aspect of sperm morphology or body size, supporting the hypothesis that post-copulatory sexual selection acts more strongly upon sperm production than upon sperm morphology. Likewise, evolutionary increases in testis size, which typically indicate increased sperm competition, are not associated with predictable changes in sperm morphology, suggesting that any effects of post-copulatory selection on sperm morphology are either weak or variable in direction across anoles. Collectively, our results suggest that sperm production is the primary target of post-copulatory sexual selection in this lineage.

Executive control and faithfulness: only long-term romantic relationships require prefrontal control

Individuals in the early stages of a romantic relationship generally express intense passionate love toward their partners. This observation allows us to hypothesize that the regulation of interest in extra-pair relationships by executive control, which is supported by the function of the prefrontal cortex, is less required in individuals in the early stages of a relationship than it is in those who are in a long-term relationship. To test this hypothesis, we asked male participants in romantic relationships to perform a go/no-go task during functional magnetic resonance imaging (fMRI), which is a well-validated task that can measure right ventrolateral prefrontal cortex (VLPFC) activity implicated in executive control. Subsequently, the participants engaged in a date-rating task in which they rated how much they wanted to date unfamiliar females. We found that individuals with higher right VLPFC activity better regulated their interest in dates with unfamiliar females. Importantly, this relationship was found only in individuals with long-term partners, but not in those with short-term partners, indicating that the active regulation of interest in extra-pair relationships is required only in individuals in a long-term relationship. Our findings extend previous findings on executive control in the maintenance of monogamous relationships by highlighting the role of the VLPFC, which varies according to the stage of the romantic relationship.

Humans as a model species for sexual selection research

Ever since Darwin, questions about humans have driven sexual selection research. While studies of other organisms are often justified as useful for improving understanding of humans, humans themselves can be useful models. Although humans present some drawbacks as model organisms (complicated societies, slow reproduction and strong ethical constraints on experimental options), humans nonetheless offer many advantages (being abundant, accessible and having detailed historical records for some populations). As an additional challenge, humans exhibit a rather puzzling combination of traits. Some traits (pair-bonding, biparental care and modest sexual dimorphism in body size) suggest selection for monogamous mating, while other traits (including sexual dimorphism in body composition and appearance) suggest selection for polygyny. Such puzzles have motivated research on other species, resulting in a rich set of comparative data that provides insights into humans and other species. Recent studies of visual trait dimorphism suggest that human appearance reflects adaptation for multi-level societies, rather than high levels of polygyny. In addition to biological traits, human cultural traits have undergone rapid evolution. Changes in subsistence strategies profoundly affect opportunities for sexual selection. The enormous variability of human behaviour and ecology provides abundant opportunities to test key hypotheses, and poses challenging puzzles for future research.

Few Nuclear-Encoded Mitochondrial Gene Duplicates Contribute to Male Germline-Specific Functions in Humans

Most of the genes encoding proteins that function in the mitochondria are located in the nucleus and are called nuclear-encoded mitochondrial genes, or N-mt genes. In Drosophila melanogaster , about 23% of N-mt genes fall into gene families, and all duplicates with tissue-biased expression (76%) are testis biased. These genes are enriched for energy-related functions and tend to be older than other duplicated genes in the genome. These patterns reveal strong selection for the retention of new genes for male germline mitochondrial functions. The two main forces that are likely to drive changes in mitochondrial functions are maternal inheritance of mitochondria and male-male competition for fertilization. Both are common among animals, suggesting similar N-mt gene duplication patterns in different species. To test this, we analyzed N-mt genes in the human genome. We find that about 18% of human N-mt genes fall into gene families, but unlike in Drosophila , only 28% of the N-mt duplicates have tissue-biased expression and only 36% of these have testis-biased expression. In addition, human testis-biased duplicated genes are younger than other duplicated genes in the genome and have diverse functions. These contrasting patterns between species might reflect either differences in selective pressures for germline energy-related or other mitochondrial functions during spermatogenesis and fertilization, or differences in the response to similar pressures.

Paternal and grandpaternal ages at conception and descendant telomere lengths in chimpanzees and humans

Telomeres are repeating DNA at chromosome ends. Telomere length (TL) declines with age in most human tissues, and shorter TL is thought to accelerate senescence. In contrast, older men have sperm with longer TL; correspondingly, older paternal age at conception (PAC) predicts longer TL in offspring. This PAC-effect could be a unique form of transgenerational genetic plasticity that modifies somatic maintenance in response to cues of recent ancestral experience. The PAC-effect has not been examined in any non-human mammals.

OBJECTIVES

Here, we examine the PAC-effect in chimpanzees (Pan troglodytes). The PAC-effect on TL is thought to be driven by continual production of sperm-the same process that drives increased de novo mutations with PAC. As chimpanzees have both greater sperm production and greater sperm mutation rates with PAC than humans, we predict that the PAC-effect on TL will be more pronounced in chimpanzees. Additionally we examine whether PAC predicts TL of grandchildren.

MATERIALS AND METHODS

TL were measured using qPCR from DNA from blood samples from 40 captive chimpanzees and 144 humans.

RESULTS

Analyses showed increasing TL with PAC in chimpanzees (p = .009) with a slope six times that in humans (p = .026). No associations between TL and grandpaternal ages were found in humans or chimpanzees-although statistical power was low.

DISCUSSION

These results suggest that sperm production rates across species may be a determinant of the PAC-effect on offspring TL. This raises the possibility that sperm production rates within species may influence the TL passed on to offspring.

Why so many sperm cells? Not only a possible means of mitigating the hazards inherent to human reproduction but also an indicator of an exaptation

Redundancy—the excess of supply over necessity—has recently been proposed for human sperm cells. However, the apparent superfluity of cell numbers may be necessary in order to circumvent the hazards, many of which can be quantified, that can occur during the transition from gametogenesis within the testes to zygosis within the female reproductive tract. Sperm cell numbers are directly related to testicular volume, and it is owing to a redundancy, and the possible exaptation, of this latter parameter that a putative excess of sperm cells is perceived.

Cuckolded Fathers Rare in Human Populations

Contemporary data of extra-pair paternity (EPP) in human populations may be biased by the use of modern contraceptives. Studies have now estimated historical EPP rates in several human populations. The observed low EPP rates challenge the idea that women routinely 'shop around' for good genes by engaging in extra-pair copulations.

Does human ejaculate quality relate to phenotypic traits?

A given man's phenotype embodies cues of his ancestral ability to effectively defend himself and his kin from harm, to survive adverse conditions, and to acquire status and mating opportunities. In this review, we explore the hypothesis that a man's phenotype also embodies cues to fertility or the probability that an ejaculate will fertilize ova. Female mate choice depends on the ability to discern the quality of a male reproductive partner through his phenotype, and male fertility may be among the traits that females have evolved to detect. A female who selects as mates males that deliver higher quality ejaculates will, on average, be more fecund than her competitors. Data on several non-human species demonstrate correlations between ejaculate quality and secondary sexual characteristics that inform female mate choice, suggesting that females may select mates in part on the basis of fertility. While the non-human literature on this topic has advanced, the human literature remains limited in scope and there is no clear consensus on appropriate methodologies or theoretical positions. We provide a comprehensive review and meta-analysis of this literature, and conclude by proposing solutions to the many issues that impede progress in the field. In the process, we hope to encourage interest and insight from investigators in other areas of human mating and reproductive biology. Am. J. Hum. Biol. 28:318-329, 2016. © 2015 Wiley Periodicals, Inc.

Mate guarding and frequent in-pair copulation in humans : Concurrent or compensatory anti-cuckoldry tactics?

Cuckoldry is an adaptive problem faced by parentally investing males of socially monogamous species (e.g., humans and many avian species). Mate guarding and frequent in-pair copulation (IPC) may have evolved as anti-cuckoldry tactics in avian species and in humans. In some avian species, the tactics are used concurrently, with the result that mate guarding behaviors and IPC frequency are correlated positively. In other avian species, the tactics are compensatory, with the result that mate guarding behaviors and IPC frequency are correlated negatively. The relationship between mate guarding and IPC frequency in humans is unknown. Avian males that use these tactics concurrently share with human males an inability to guard a female partner continuously during her peak fertile period. We hypothesized, therefore, that men's mate guarding and IPC frequency function as concurrent anti-cuckoldry tactics, resulting in a positive correlation between them. Study 1 (n=305) secured men's self-reports of mate guarding and IPC frequency. Study 2 (n+367) secured women's reports of their partners' mate guarding and IPC frequency. The concurrent tactics hypothesis was supported in both studies: Men's mate guarding and IPC frequency are correlated positively, and this association is not attributable to male age, female age, relationship satisfaction, relationship length, or time that the couple spends together. The Discussion section addresses potential limitations of this research and future research directions.

Three hundred years of low non-paternity in a human population

When cuckoldry is frequent we can expect fathers to withhold investment in offspring that may not be theirs. Human paternal investment can be substantial and is in line with observations from tens of thousands of conceptions that suggest that cuckoldry is rare in humans. The generality of this claim seems to be in question as the rate of cuckoldry varies across populations and studies have mostly been on Western populations. Two additional factors complicate our conclusions, (1) current estimates of the rate of cuckoldry in humans may not reflect our past behaviour as adultery can be concealed by the use of contraceptives; and (2) it is difficult to obtain samples that are random with respect to their paternity certainty. Studies that combine genealogies with Y-chromosome haplotyping are able to circumvent some of these problems by probing into humans' historical behaviour. Here we use this approach to investigate 1273 conceptions over a period of 330 years in 23 families of the Afrikaner population in South Africa. We use haplotype frequency and diversity and coalescent simulations to show that the male population did not undergo a severe bottleneck and that paternity exclusion rates are high for this population. The rate of cuckoldry in this Western population was 0.9% (95% confidence interval 0.4-1.5%), and we argue that given the current data on historical populations we have to conclude that, at least for Western human populations, cuckoldry rate is probably in the range of 1%.

Polymorphisms in inhibin α gene promoter associated with male infertility

Inhibins play important roles in normal gonadal function, including regulation of proliferation, differentiation, and steroidogenesis of Leydig and Sertoli cells via paracrine and autocrine processes. In adult males, circulating inhibin levels are correlated with fertility by regulating the number of Sertoli cells, total sperm count, and testicular volume. Given this important role, inhibin-α subunit (INHA) is a strong candidate gene in male fertility. However, limited data regarding the association of polymorphisms of INHA with male fertility are available. This study was based on the hypothesis that polymorphisms in the promoter of INHA are associated with male fertility. Han Chinese patients with non-normozoospermia (n=153) and normozoospermia (n=72) from Northern China were screened, and genotypes were analyzed by polymerase chain reaction-restriction fragment length polymorphism after INHA promoter was amplified. Statistical analysis results revealed a significant difference in the allele frequency of INHA promoter between males with non-normozoospermia and normozoospermia. For c.-124G>A, males carrying c.-124GG genotype and c.-124GA genotype showed an increased risk of non-normozoospermic syndrome. For c.-16C>T polymorphism, no significant difference in allele frequency was observed between the two groups. Therefore, the haplotype AC possibly displayed a considerable reduced risk of non-normozoospermic syndrome.

Role of Partner Novelty in Sexual Functioning: A Review

This review investigates whether sexual desire and arousal decline in response to partner familiarity, increase in response to partner novelty, and show differential responding in men and women. These questions were considered through the perspective of two leading evolutionary theories regarding human mating strategies: sexual strategies theory and attachment fertility theory. The hypotheses emerging from these theories were evaluated through a critical analysis of several areas of research including habituation of arousal to erotic stimuli, preferences regarding number of sexual partners, the effect of long-term monogamous relationships on sexual arousal and desire, and prevalence and risk factors associated with extradyadic behavior. The current literature best supports the predictions made by sexual strategies theory in that sexual functioning has evolved to promote short-term mating. Sexual arousal and desire appear to decrease in response to partner familiarity and increase in response to partner novelty in men and women. Evidence to date suggests this effect may be greater in men.

Sperm competition in humans: mate guarding behavior negatively correlates with ejaculate quality

In species where females mate with multiple males, the sperm from these males must compete to fertilise available ova. Sexual selection from sperm competition is expected to favor opposing adaptations in males that function either in the avoidance of sperm competition (by guarding females from rival males) or in the engagement in sperm competition (by increased expenditure on the ejaculate). The extent to which males may adjust the relative use of these opposing tactics has been relatively neglected. Where males can successfully avoid sperm competition from rivals, one might expect a decrease in their expenditure on tactics for the engagement in sperm competition and vice versa. In this study, we examine the relationship between mate guarding and ejaculate quality using humans as an empirical model. We found that men who performed fewer mate guarding behaviors produced higher quality ejaculates, having a greater concentration of sperm, a higher percentage of motile sperm and sperm that swam faster and less erratically. These effects were found independent of lifestyle factors or factors related to male quality. Our findings suggest that male expenditure on mate guarding and on the ejaculate may represent alternative routes to paternity assurance in humans.

Low historical rates of cuckoldry in a Western European human population traced by Y-chromosome and genealogical data

Recent evidence suggests that seeking out extra-pair paternity (EPP) can be a viable alternative reproductive strategy for both males and females in many pair-bonded species, including humans. Accurate data on EPP rates in humans, however, are scant and mostly restricted to extant populations. Here, we provide the first large-scale, unbiased genetic study of historical EPP rates in a Western European human population based on combining Y-chromosomal data to infer genetic patrilineages with genealogical and surname data, which reflect known historical presumed paternity. Using two independent methods, we estimate that over the last few centuries, EPP rates in Flanders (Belgium) were only around 1–2% per generation. This figure is substantially lower than the 8–30% per generation reported in some behavioural studies on historical EPP rates, but comparable with the rates reported by other genetic studies of contemporary Western European populations. These results suggest that human EPP rates have not changed substantially during the last 400 years in Flanders and imply that legal genealogies rarely differ from the biological ones. This result has significant implications for a diverse set of fields, including human population genetics, historical demography, forensic science and human sociobiology.

Does mating prevent monogamous males from seeking other females? A study in prairie voles (Microtus ochrogaster)

Male prairie voles form pair bonds under laboratory conditions, but show a variety of mating tactics in nature. We tested them in the laboratory to determine if their decision to reproduce with a single or multiple females is related to how they process sensory information from females. Three groups of mated males were tested for their attentiveness toward two females and their odors. Males given a choice to investigate a box holding their mate or a box holding a sexually receptive female spent more time with the box of the sexually receptive female than that of their mate. Similar results were found when females were removed and replaced by their odors. However, males did not attend preferentially to the sexually receptive female under all circumstances. When given a choice between a sexually unreceptive and a sexually receptive female, males did not display a difference in their attentiveness. Furthermore, males tested in presence of their mate were more attentive to the odor of the sexually receptive female than males tested in presence of a sexually unreceptive female. The data suggest that access to the mate's sensory cues may influence male's decision to seek females other than his mate.

Testicular volume is inversely correlated with nurturing-related brain activity in human fathers

Despite the well-documented benefits afforded the children of invested fathers in modern Western societies, some fathers choose not to invest in their children. Why do some men make this choice? Life History Theory offers an explanation for variation in parental investment by positing a trade-off between mating and parenting effort, which may explain some of the observed variance in human fathers' parenting behavior. We tested this hypothesis by measuring aspects of reproductive biology related to mating effort, as well as paternal nurturing behavior and the brain activity related to it. Both plasma testosterone levels and testes volume were independently inversely correlated with paternal caregiving. In response to viewing pictures of one's own child, activity in the ventral tegmental area--a key component of the mesolimbic dopamine reward and motivation system--predicted paternal caregiving and was negatively related to testes volume. Our results suggest that the biology of human males reflects a trade-off between mating effort and parenting effort, as indexed by testicular size and nurturing-related brain function, respectively.

Selflessness is sexy: reported helping behaviour increases desirability of men and women as long-term sexual partners

BACKGROUND

Despite its short-term costs, behaviour that appears altruistic can increase an individual's inclusive fitness by earning direct (selfish) and/or indirect (kin-selected) benefits. An evolved preference for other-regarding or helping behaviour in potential mates has been proposed as an additional mechanism by which these behaviours can yield direct fitness benefits in humans.

RESULTS

We asked 32 heterosexual women and 35 heterosexual men to rate the attractiveness of members of the opposite sex in the presence and the absence of information about helping behaviours. Reports of helping behaviour were associated with a significant increase in the attractiveness of both men and women as potential long-term sexual partners. Altruism also increased the attractiveness of men as potential partners for short-term flings, but to a lesser extent than when the same men were being considered for long-term relationships. Altruism did not affect the attractiveness of women as partners for short-term flings.

CONCLUSIONS

Our results unite two important areas of evolutionary theory - social evolution and sexual selection - and extend the list of means by which helping behaviours, which appear at first glance to be costly to the actor, can in fact earn direct fitness benefits. Helping behaviours may be attractive because they signal 'good genes' and/or because they are perceived as a signal of likely provision of non-genetic benefits (e.g. parental care). Exactly why helping behaviours in a non-mating context might be attractive to potential mates, and whether they are honest signals of mate quality, remains to be elucidated.

Evolution of life history and behavior in Hominidae: towards phylogenetic reconstruction of the chimpanzee-human last common ancestor

The origin of the fundamental behavioral differences between humans and our closest living relatives is one of the central issues of evolutionary anthropology. The prominent, chimpanzee-based referential model of early hominin behavior has recently been challenged on the basis of broad multispecies comparisons and newly discovered fossil evidence. Here, we argue that while behavioral data on extant great apes are extremely relevant for reconstruction of ancestral behaviors, these behaviors should be reconstructed trait by trait using formal phylogenetic methods. Using the widely accepted hominoid phylogenetic tree, we perform a series of character optimization analyses using 65 selected life-history and behavioral characters for all extant hominid species. This analysis allows us to reconstruct the character states of the last common ancestors of Hominoidea, Hominidae, and the chimpanzee-human last common ancestor. Our analyses demonstrate that many fundamental behavioral and life-history attributes of hominids (including humans) are evidently ancient and likely inherited from the common ancestor of all hominids. However, numerous behaviors present in extant great apes represent their own terminal autapomorphies (both uniquely derived and homoplastic). Any evolutionary model that uses a single extant species to explain behavioral evolution of early hominins is therefore of limited use. In contrast, phylogenetic reconstruction of ancestral states is able to provide a detailed suite of behavioral, ecological and life-history characters for each hypothetical ancestor. The living great apes therefore play an important role for the confident identification of the traits found in the chimpanzee-human last common ancestor, some of which are likely to represent behaviors of the fossil hominins.

Women's attractiveness changes with estradiol and progesterone across the ovulatory cycle

In many species, females are more sexually attractive to males near ovulation. Some evidence suggests a similar pattern in humans, but methodological limitations prohibit firm conclusions at present, and information on physiological mechanisms underlying any such pattern is lacking. In 202 normally-cycling women, we explored whether women's attractiveness changed over the cycle as a function of two likely candidates for mediating these changes: estradiol and progesterone. We scheduled women to attend one session during the late follicular phase and another during the mid-luteal phase. At each session, facial photographs, voice recordings and saliva samples were collected. All photographs and voice recordings were subsequently rated by men for attractiveness and by women for flirtatiousness and attractiveness to men. Saliva samples were assayed for estradiol and progesterone. We found that progesterone and its interaction with estradiol negatively predicted vocal attractiveness and overall (facial plus vocal) attractiveness to men. Progesterone also negatively predicted women's facial attractiveness to men and female-rated facial attractiveness, facial flirtatiousness and vocal attractiveness, but not female-rated vocal flirtatiousness. These results strongly suggest a pattern of increased attractiveness during peak fertility in the menstrual cycle and implicate estradiol and progesterone in driving these changes.

Artificially selected human sperm morphology after swim-up processing

The swim-up technique is a clinical practice used to select highly motile sperm cells from patient ejaculates to use in assisted fertilization. The aim of this study was to investigate whether the length of different sperm-cell components is related to gamete function. Thus, we explored whether swim-up technique selects for longer sperm cells than mean sperm cells from unprocessed ejaculates. Sperm midpiece, tail endpiece, and total length were measured before and after the swim-up selection by means of contrast-phase and electron microscopy. Correlations between sperm dimensions, sperm motility, and sperm concentration were also investigated. Swim-up selected cells with longer midpiece compared with the unprocessed fractions (5.8 μm (CI 5.52–6.16 μm) vs. 5.3 μm (CI 4.97–5.61 μm), p < 0.05) and shorter tail endpiece (7.8 μm (CI 7.11–8.44 μm) vs. 8.5 μm (CI 7.81–9.14 μm), p < 0.05 after meta-analysis), whereas no effect of swim-up selection was detected on the total sperm cell length. Individuals producing high sperm concentrations had longer sperm midpiece than had men producing lower sperm concentrations. It is concluded that short sperm flagellar tips with long midpieces may be used as biomarkers in infertility therapy.

Comparing pedigree graphs

Pedigree graphs, or family trees, are typically constructed by an expensive process of examining genealogical records to determine which pairs of individuals are parent and child. New methods to automate this process take as input genetic data from a set of extant individuals and reconstruct ancestral individuals. There is a great need to evaluate the quality of these methods by comparing the estimated pedigree to the true pedigree. In this article, we consider two main pedigree comparison problems. The first is the pedigree isomorphism problem, for which we present a linear-time algorithm for leaf-labeled pedigrees. The second is the pedigree edit distance problem, for which we present (1) several algorithms that are fast and exact in various special cases, and (2) a general, randomized heuristic algorithm. In the negative direction, we first prove that the pedigree isomorphism problem is as hard as the general graph isomorphism problem, and that the sub-pedigree isomorphism problem is NP-hard. We then show that the pedigree edit distance problem is APX-hard in general and NP-hard on leaf-labeled pedigrees. We use simulated pedigrees to compare our edit-distance algorithms to each other as well as to a branch-and-bound algorithm that always finds an optimal solution.

Intra-individual conflicts between autosomal and X-linked altruistic genes: evolutionary perspectives of sex-specific grandmothering

Alloparental care by females toward their grandoffspring can evolve by kin selection. Previous theoretical studies predicted that selection favors autosomal and X-chromosomal genes, causing altruism toward maternal grandoffspring and paternal granddaughters, respectively, and two corresponding types of biased grandparental investment are suggested by empirical studies on human populations. Using discrete-time two-locus-two-allele models, I examined a possible conflict between the autosomal and the X-chromosomal altruistic genes over the carrier female's time and resources. This conflict is expected to occur when each grandmother has access to only maternal or paternal grandchildren as a result of her residence situation. The conditions under which each or both kinds of altruistic genes evolve (against non-altruistic genes) mainly represent the conflicting relationship between the autosomal and X-chromosomal altruistic genes. In addition, depending on the settings, the models exhibit bistable or periodic behaviors, and one type of gene can be considered parasitic in the latter behavior. On the whole, the results suggest that the X-chromosomal altruistic genes rather than the autosomal ones exhibit more difficulty increasing or fixing with this kind of conflict.

Religion as a means to assure paternity

The sacred texts of five world religions (Buddhism, Christianity, Hinduism, Islam, and Judaism) use similar belief systems to set limits on sexual behavior. We propose that this similarity is a shared cultural solution to a biological problem: namely male uncertainty over the paternity of offspring. Furthermore, we propose the hypothesis that religious practices that more strongly regulate female sexuality should be more successful at promoting paternity certainty. Using genetic data on 1,706 father-son pairs, we tested this hypothesis in a traditional African population in which multiple religions (Islam, Christianity, and indigenous) coexist in the same families and villages. We show that the indigenous religion enables males to achieve a significantly (P = 0.019) lower probability of cuckoldry (1.3% versus 2.9%) by enforcing the honest signaling of menstruation, but that all three religions share tenets aimed at the avoidance of extrapair copulation. Our findings provide evidence for high paternity certainty in a traditional African population, and they shed light on the reproductive agendas that underlie religious patriarchy.

Visual judgments of kinship: an alternative perspective

Following other researchers, we investigated the premise that visual judgment of kinship might be modelled as a signal-detection task, strictly related to similar facial features. We measured subjects' response times to face-pair stimuli while they performed visual judgments of kinship, similarity, or dissimilarity, and examined some priming effects involved. Our results show that kinship judgment takes longer on average than either similarity or dissimilarity judgment-which is compatible with existing models, yet might also suggest that kinship judgments are of a more complex character. In our priming study we observed selective suppression/enhancement of the efficacy of dissimilarity judgments whenever they followed similarity and kinship judgments. This finding confounds the notion, inherent in previous models, of resemblance cues signalling for kinship, since similarity and dissimilarity cannot be considered just as opposite concepts, and observed priming effects need to be explicitly modelled, including dissimilarity cues. To model kinship judgments across faces that are perceived as dissimilar, a new framework may be required, perhaps accepting the perspective of a task-driven use of the visual cues, modulated by experience and cultural conditioning.

The Evolution of Female Orgasm: Adaptation or Byproduct?

Do women experience orgasm because this trait was shaped by natural selection to augment female fitness? Or are women merely the lucky recipients of developmental patterns favored by selection to produce orgasm in males? A recent and widely publicized book by Elisabeth Lloyd (2005a) contends that there is insufficient evidence to validate any of the adaptive explanations yet proposed for female orgasm. We agree. But our reading of the data differs from Lloyd's. In this essay, we outline why, unlike Caton (2006), whose review of Lloyd's book appeared previously in this journal, we are not persuaded by Lloyd's argument that female orgasm is a nonadaptive byproduct of orgasm in men. We hold this view because we disagree with the criteria Lloyd uses to evaluate evolutionary hypotheses, and because we believe Lloyd defines female orgasm too narrowly, ignoring critical information about its affective aspects.

Low pitched voices are perceived as masculine and attractive but do they predict semen quality in men?

Women find masculinity in men's faces, bodies, and voices attractive, and women's preferences for men's masculine features are thought to be biological adaptations for finding a high quality mate. Fertility is an important aspect of mate quality. Here we test the phenotype-linked fertility hypothesis, which proposes that male secondary sexual characters are positively related to semen quality, allowing females to obtain direct benefits from mate choice. Specifically, we examined women's preferences for men's voice pitch, and its relationship with men's semen quality. Consistent with previous voice research, women judged lower pitched voices as more masculine and more attractive. However men with lower pitched voices did not have better semen quality. On the contrary, men whose voices were rated as more attractive tended to have lower concentrations of sperm in their ejaculate. These data are more consistent with a trade off between sperm production and male investment in competing for and attracting females, than with the phenotype-linked fertility hypothesis.

The selfish grandma gene: the roles of the x-chromosome and paternity uncertainty in the evolution of grandmothering behavior and longevity

When considering inclusive fitness, it is expected that individuals will provide more care towards those with whom they are more closely related. Thus, if a selfish X-linked genetic element influenced care giving, we would expect care giving to vary with X-relatedness. Recent studies have shown that X-chromosome inheritance patterns may influence selection of traits affecting behavior and life-history. Sexually antagonistic (SA) zygotic drive could encourage individuals to help those with whom they are more likely to share genetic material at the expense of other relatives. We reanalyze previously reported data in light of this new idea. We also evaluate the effects of paternity uncertainty on SA-zygotic drive. Our evidence suggests that human paternal discrepancy is relatively low. Using published models, we find the effects of paternal discrepancy do not override opportunity for selection based on X-relatedness. Based on these results, longevity and grandmothering behaviors, including favoritism, may be more heavily influenced by selection on the X-chromosome than by paternity uncertainty.